1. Field of the Invention
The present invention relates to an apparatus and a method for A/D conversion in a digital video system, and more particularly, to an apparatus and a method for A/D conversion in a digital video system to minimize a quantization error without increasing the number of quantization bits during the A/D conversion, thereby improving an image quality. The present application is based on Korean Patent Application No. 2002-47753, which is incorporated herein by reference.
2. Description of the Prior Art
A digital video system such as a digital TV (DTV) requires an analog to digital conversion (A/D conversion) for displaying an image on a screen.
The A/D conversion is a process of converting a continuously changing analog video signal or audio signal to a discrete digital signal. A sampling frequency and the number of quantization bits in the A/D conversion affect an image quality, a sound quality, and an amount of information of the digital video system. That is, an increased sampling frequency guarantees an excellent video and audio quality because a denser sampling enables a high frequency to be reproduced. However, in proportion to the increased sampling frequency, an amount of the information to be transmitted and recorded typically increases as much as the sampling frequency.
The number of the quantization bits corresponding to one sample also affects the video and sound quality, and the amount of the information. For example, a mosaic image is widely used for purposes such as rights of portrait protection, and is expressed too fuzzy for a viewer to recognize an actual image. That is caused by the decreased amount of the quantization bits in a digital signal processing.
The quantization in the A/D conversion is the process of manipulating continuously distributed sample values to represent a discrete set of numerical values, which inevitably causes the loss of information. For example, if a quantization step width is 1V, an output value is obtained by rounding-off an input value in a manner that an input signal value of 0.5 V and below becomes 0V, 0.5V˜1.5V becomes 1V, and 1.5V˜2.5V becomes 2V. That is, when the continuous analog signal is converted to the discrete signal in a stepped fashion, there inevitably occurs an error between the input signal and the output signal. This error is called a quantization error. Also, the quantization error is referred to as a quantization noise, because it appears in the form of a noise with respect to a signal.
The quantization error does not exceed a half of the quantization step width, and is reduced as the quantization step width is decreased. In other words, decreasing the quantization step width increases the number of the quantization bits. However, since there is a limit to increasing the number of the quantization bits, the quantization error cannot be reduced to substantially zero(0). That is, the more the quantization bits are increased, the narrower the quantization step width becomes, which results in the improved video and audio quality, but also the increased amount of the information in proportion to the number of the quantization bits. Accordingly, the number of quantization bits has to be increased properly.
Due to this technological restriction, acoustic apparatuses, which have a relatively smaller amount of information, have been digitalized ahead of image apparatuses. A typical digital acoustic apparatus such as a CD has a sampling frequency of 44.1 kHz and quantization bits of 16-bits. In the case of a motion picture signal, a bandwidth of which is 200 times as much as an audio signal, if the number of quantization bits is equal to or greater than 16-bits, the amount of the information reaches astronomical figures, and thus, it is practically impossible to convert or record an image signal to a digital code. Accordingly, the number of quantization bits in the digital video system has to be determined in consideration of costs, visual characteristics of human eyes, and the level of image quality personally evaluated by viewers with respect to an actual screen as having no problem. Generally, the number of quantization bits is 8-bits or 10-bits, and recently, a digital processor camera of 12-bits has been developed.
The resolution of 8-bits is 28 level (=256) at a maximum, the resolution of 10-bits is 210 level (=1024) at a maximum, and the resolution of 12-bits is 212 level (=4096) at a maximum. The 8-bit system has the quantization step width larger than that of a 10-bit system so that an amount of the information to be processed is much smaller, but the 8-bit system has a difficulty in depicting an obliquely shaped object such as an oblique line or an arc shape as precisely as an analog system.
Accordingly, in order to obtain high video and audio quality in the digital video system, it is required to increase the number of the quantization bits and thus decrease the quantization step width. However, since increasing the number of quantization bits results in an increased amount of information, there are problems of complicated circuit structure and increased fabrication costs.
For the above reason, the digital video system generally employs the 8-bit system. In this case, the digital video system uses an 8-bit quantized level with respect to a luminance (Y) signal and a color difference signal (Cb, Cr) that are required to express a picture element or a pixel corresponding to one point in a screen, as well as quantization bits of 8-bits.
Accordingly, an A/D conversion apparatus of a digital video system is required to minimize the quantization error and thus improve the image quality without having to increase the number of the quantization bits during the A/D conversion and causing an increased complexity of circuit requirements or fabrication cost, when the quantization level of the luminance (Y) or the color difference signal (Cb, Cr) for expressing the pixel is greater than 8-bits.